1. Field of the Invention
This invention relates to measuring and testing of corrosion processes, and it relates more particularly to an electrochemical cell system for measuring atomic hydrogen created by the corrosion of ferrous metals.
2. Description of the Prior Art
It is often desirable to determine the rates at which ferrous metals corrode within a corrodant, such as a corrosive aqueous liquid. For example, corrosion inhibitors are added to aqueous liquids to reduce the corrosion of exposed metals. Instruments are used to measure the rates at which these metals corrode so that the effectiveness of inhibitor addition can be determined. One measurement of the rate of corrosion upon ferrous metals involves the determination of the amount of atomic hydrogen created by the corrosion reaction of a ferrous metal exposed to a corrodant. For example, a steel sidewall of a pipeline carrying a corrodant, such as hydrogen sulfide in water, has a corrosion reaction creating atomic hydrogen which defuses through the sidewall and released exteriorly as molecular hydrogen gas. However, the molecular or atomic hydrogen inside the sidewall can oftentimes build up to a sufficient pressure causing physical injury such as blistering and rupturing of the sidewall metal.
Various measurement systems have been proposed for the measurement of the hydrogen produced by the corrosion reaction. For this purpose, a probe may be inserted through the sidewall of the pipeline and arranged to measure the molecular hydrogen gas pressure buildup within the probe. For this purpose, the probe has a ferrous metal body in which there is formed a cavity. The corrosion reaction produced by the corrodant surrounding the probe causes molecular hydrogen gas to accumulate within the cavity. A pressure guage mounted atop the probe indicates the actual pressure of hydrogen gas accumulating within the cavity. For example, in very active corrodants, the pressure buildup of such a probe can reflect hydrogen gas accumulations within the cavity from an initial 15 psi to about 100 psi or more within a 24-hour period. The probe carries a manual venting valve so that the pressure can be released from the cavity when the pressure limits of the guage are reached. Thus, this type of hydrogen measurement probe must be employed in a supervised manner wherein the operator can periodically record the readings of the probe and also vent hydrogen gas as necessary to prevent the destruction of the pressure guage. This type of hydrogen measurement probe is simple and relatively inexpensive but has not found extensive utilization in the industry because of the requirement for relatively constant supervision.
Another type of hydrogen measurement probe avoids the supervision problem but employs a sophisticated gas ionization instrumentation principle. In this probe, the hydrogen gas is vented in a relatively continuous manner from the cavity within the probe body. The vented gas flows through an ionization chamber and detector sensor whose output is measured upon a scalar instrument indicating both total gas volume and rate of gas flow. This probe and readout instrumentation is relatively accurate, very expensive and dependable, but requires careful calibration and complicated installation. Also, this probe is relatively too delicate to use unattended within oil fields, refineries and chemical plants.
Hydrogen gas amounts diffusing from a ferrous metal may be determined by an electrochemical method. In particular, this method measures the amount of hydrogen by electrochemical conversion of hydrogen gas to hydrogen ions. Potentiostatic or coulometric circuitry can provide the required current to affect the conversion from gas to ion forms of hydrogen and this current is directly proportional to the hydrogen permeation rate. An example of such a measurement is through the use of a "barnacle electrode" described in a publication, "Hydrogen Embrittlement Resulting from Corrosion, Cathodic Protection, and Electroplating," prepared by E. Gileadi and submitted by the Electrochemistry Laboratory of the University of Pennsylvania to the Naval Air Engineering Center, U.S. Navy in September 1966.
The "barnacle electrode" was developed for use with individual steel specimens and utilizes the electrochemical method for determination of the permeation rate of hydrogen. A small section is taken of the metal to be tested, and this metal specimen is cleaned and then placed into a glass electrolytic cell containing counter (auxiliary) and reference electrodes. The glass cell is filled on one side of the specimen with an electrolyte which may be an alkaline solution (0.2 N NaOH). The other side of the specimen is exposed to a source of hydrogen. If desired, a thin coating of palladium on the diffusion side of the test specimen may be employed to avoid anodic dissolution of the base metal by the electrolyte. The test specimen forms the test electrode of the system. The potential between the test and the reference electrodes is set at the required value so that substantially all of the hydrogen atoms are converted immediately to hydrogen ions upon entering into the electrolyte from the test specimen. The current flow required for this purpose is determined employing the counter electrode (which may be a silver silver oxide type). An integration of the current as a function of time produces the total amount of hydrogen diffusing through the specimen. Alternatively, the concentration of hydrogen diffusing from the specimen may be obtained from the shape of the current-time transient obtained from the cell. This technique requires a knowledge of the diffusion coefficient of hydrogen in the metal being tested. However, in any event, a small individual test specimen of the ferrous metal desired to be measured for hydrogen gas diffusion must be obtained and placed directly into the cell as an active element. As will be apparent, such an arrangement is impractical to be used in operating plants or in other than laboratory locations.
In many commercial installations, such as oil refineries and chemical plants, the integrity of the piping should not be disturbed for the placement of a hydrogen measuring probe system. This is especially true of the electric power generating plants employing atomic energy sources. In many of these plants, very pure water is employed for heat transfer or as a moderating fluid. In addition, the piping contains relatively high pressures and temperatures with a limited number of entry points which are constantly monitored so that any possible escape of radioactive material can be avoided. As is apparent, corrosion in such piping seriously contaminates the water being employed for the mentioned purposes. In particular, corrosion products entering the water would become radioactive. This undesired corrosion product would then become a source of atomic contamination throughout the electric power generating system. The number of entry points into the piping for introduction of corrosion measurement probes is extremely limited and of serious design nature. However, the corrositivity of the water contained in the piping must be measured to insure safe operation. It would be most desirable to employ a hydrogen measurement probe system with the water piping of a type not requiring special openings into the pipe, disrupting the integrity of the piping, or in any way jeopardizing the strength of the piping through induced external surface corrosion.
The hydrogen patch cell of the present invention provides a simplicity in construction and use in any environment without requiring a disruption of the integrity of piping or otherwise exposing the system to injury. In addition, this hydrogen patch cell produces the measurement of hydrogen diffusion in metallic walls with great utility and high accuracy equal to the best known prior art electrochemical methods.